Electroluminescent lamps have a phosphor-bearing dielectric layer between two electrodes. A front electrode is provided which is a transparent conductor, such as indium tin oxide, while a back electrode is provided which can be a non-transparent conductor. When the two electrodes are maintained at different potentials, the phosphor-bearing dielectric layer emits light which radiates through the transparent electrode and provides a light source for the lamp. Electroluminescent displays are similar to lamps with the exception that multiple pairs of electrodes are used so that selected regions of the displays can be lighted.
There are two types of electroluminescent lamps, thin film and thick film lamps. Thin film lamps are usually formed by deposition of the electrodes and the circuit architecture onto a glass substrate. U.S. Pat. Nos. 3,153,167 and 3,254,254 teach two such lamps. To extend the life of the resulting lamp, the lamp is encapsulated in glass to preserve the integrity of the phosphor layer. The encapsulation protects the phosphor layer from the deleterious effects of moisture. Metal contacts and leads are employed to connect voltage sources to the electrodes and these contacts and leads can be sealed into the encapsulating glass by packing the electrode with glass beads and sintering the beads to form a seal between the contacts and leads and the encapsulating glass. Since the contacts and leads can be fused into the encapsulating glass, they can be arranged at will and thus, a lamp having all contacts within the footprint of the lamp can be readily attained. While thin film lamps have many advantages, they are difficult to manufacture, will frequently fail if bent, and are relatively heavy. Some of these problems associated with thin film lamps have been cured by thick film lamps.
Thick film lamps usually employ a polymer film such as a MYLAR.RTM. film as a substrate rather than a glass plate. The architecture for the lamp is either applied by printing onto the base film, or by rolling the additional layers forming the architecture onto the base film. U.S. Pat. Nos. 5,045,755 and 5,120,618 are examples of typical thick film lamps. Thick film lamps are encapsuled by being sandwiched between plastic sheets which are sealed around the periphery of the lamp to avoid the deleterious effects of moisture on the phosphor layer. While thick film lamps have overcome many of the problems associated with thin film lamps such as weight and their fragile nature, the encapsulation in envelopes formed from laminated sheets necessitates the use of side electrical connectors that must be sandwiched between the sheets and substantially limits where connections can be made.
A partial solution to the problem of moisture without encapsulation is offered by U.S. Pat. Nos 4,775,964 which provides limited protection for phosphor layers of an electroluminescent lamp for a watch face. The '964 patent employs a layer of barium titanate over the architecture to resist moisture. While the barium titanate reduces the exposed area of the phosphor layer which is subject to moisture, the phosphor will still be subject to the effects of moisture at the edges of the watch face, around the contact of the lamp, and in the vicinity of a hole which is punched through the watch face to accommodate a shaft on which the watch hands rotate. Thus, to avoid these problems, it would be necessary to have a sealed watch housing or to employ an envelope such as taught in U.S. Pat. No. 4,743,801, the latter not being practical since not only do the contacts lie outside the footprint of the lamp but also the lamps must be pierced to allow a shaft to pass therethrough.
Thus, there is a need for a thick film lamp and display where there is freedom in the placement of contacts for the lamp or display within the footprint of the lamp or display while maintaining the integrity of the seal protecting the phosphor layer.